To test this hypothesis, Ivan Nagelkerken of the University of Adelaide in Australia and his colleagues decided to focus on the snapping shrimp, a marine animal with snaps that are among the loudest sounds in the ocean (the loudest-known being sperm-whale clicks).

They recorded sounds at three natural CO2 vents, two in Italian waters and one next to New Zealand, where pH levels range from standard current levels all the way up to those expected in oceans by the end of the century.

To examine how the animals might alter their sound in future oceans, they also exposed snapping shrimps to varying levels of pH in the lab for two to three months.

When they analysed recordings from the vents, they found that sound levels were much lower in more acidic waters.

Lab tests also revealed that both sound intensity and the number of snaps were reduced in the more acidic waters compared with typical pH levels today. It’s still not clear why the shrimp reduce their snapping behaviour in less alkaline waters.

Less snappy

The findings mean that in the near future, fish and invertebrate larvae that rely on these sounds to guide them from the open ocean to reefs where they can settle and develop could be affected.

“They may become lost at sea or take longer to pick up these cues as they drift with ocean currents,” says Nagelkerken. “This may reduce the number of larvae that safely arrive at coastal reefs, with consequences for the replenishment and sustainability of their populations.”

His team has already shown that larvae do not move towards the altered soundscape. However, they do continue to develop, suggesting that they could settle away from their natural habitats, where conditions may not be optimal.

Ashlee Lillis of Woods Hole Oceanographic Institution in Massachusetts, who is studying factors that affect variations in snapping-shrimp sounds in estuaries, thinks changes in other environmental factors, such as water temperature, may also be affecting the snapping behaviour.

“We know very little about the environmental drivers of their sound production, so it is difficult to predict how climate and ocean-pH scenarios may alter it in the future,” she says.

It is also possible that a higher level of acidity is affecting shrimp habitats – for example, by reducing the amount of algae they rely on for shelter, resulting in fewer shrimps and thus less sound.

Nagelkerken thinks that limiting other stressors, such as overfishing or habitat loss, could partially mitigate the impacts of acidification. “It is usually a combination of many stressors that is most detrimental and leads to the rapid collapse of ocean ecosystems,” he says.